U.S. patent number 4,518,259 [Application Number 06/401,754] was granted by the patent office on 1985-05-21 for light guide reflectometer.
This patent grant is currently assigned to Eastman Kodak Company. Invention is credited to John W. Ward.
United States Patent |
4,518,259 |
Ward |
May 21, 1985 |
Light guide reflectometer
Abstract
There is disclosed a reflectometer comprising a one-piece molded
housing that includes a radiation guide, and a source means and
detector means contained within the housing, for the analysis of a
test element. The guide, source means and detector means are
disposed so that the detector means detects reflectance from the
test element that is substantially free of specular
reflectance.
Inventors: |
Ward; John W. (Springwater,
NY) |
Assignee: |
Eastman Kodak Company
(Rochester, NY)
|
Family
ID: |
23589101 |
Appl.
No.: |
06/401,754 |
Filed: |
July 26, 1982 |
Current U.S.
Class: |
356/446;
250/227.29 |
Current CPC
Class: |
G01N
21/474 (20130101) |
Current International
Class: |
G01N
21/47 (20060101); G01N 021/47 () |
Field of
Search: |
;356/445-448,236
;250/227 ;362/31,32 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: McGraw; Vincent P.
Attorney, Agent or Firm: Schmidt; Dana M.
Claims
What is claimed is:
1. In a reflectometer adapted for measuring non-uniform
distribution of density and comprising source means for generating
radiation suitable to illuminate a test element, detector means for
detecting radiation reflected from such test element, and one piece
molded housing means in which said source means and said detector
means are mounted, said housing means including as portions
integral therewith, means for predeterminedly positioning such test
element and optically transmissive radiation guide means
constructed to guide said radiation from said source means to be
positioned test element,
the improvement wherein (i) said source means includes means for at
least partially collimating said illuminating radiation,
(ii) said radiation guide means includes an internally reflective
surface constructed and located to direct said beam to illuminate a
selected portion of a positioned test element, and
(iii) said detector means is located with respect to said
positioning means, said source means and said reflecting surface so
that the test element is illuminated by a whole spot of light that
is disposed approximately centered above said detector means, and
said detector means receives from an illuminated test element only
radiation diffusely reflected from the illuminated test
element.
2. In a reflectometer adapted for measuring non-uniform
distribution of density and comprising source means for generating
illuminating radiation; detector means for detecting radiation
reflected from a test element; one piece molded housing means in
which said source means and said detector means are mounted, said
housing means including as portions integral therewith, a support
for such test element and optically transmissive radiation guide
means disposed between said source means and said test element
support constructed to direct said illuminating radiation onto said
supported test element;
the improvement wherein (i) said source means is constructed to
deliver at least a partially collimated beam of radiation,
(ii) said guide means includes an internally reflective surface
optically disposed to direct said beam to illuminate a selected
portion of said test element, and
(iii) said detector means is disposed with respect to said support,
said source means and said reflective surface so that the test
element is illuminated by a whole spot of light that is disposed
approximately centered above said detector means, and said detector
means receives from said illuminated element only reflected
radiation substantially free of specular reflectance of said
beam.
3. A reflectometer as defined in claim 1 or 2, and further
including means for trapping specular reflectance emanating from an
illuminated test element.
4. A reflectometer as defined in claim 3, wherein said trapping
means includes an inactivated source means for generating
illuminating radiation, a portion of said guide means being
disposed to direct said specular reflectance to said inactivated
source means.
5. A reflectometer as defined in claim 1 or 2, wherein said
reflective surface comprises a generally planar, smooth exterior
surface of said guide means, disposed to reflect said illuminating
radiation received from said source means.
6. A reflectometer as defined in claim 1 or 2, wherein said source
means includes a lens that directs said illuminating radiation as
said partially collimated beam.
7. A reflectometer as defined in claim 1 or 2, wherein said source
means is mounted wholly within said housing means so as to minimize
the thickness of said reflectometer.
8. A reflectometer as defined in claim 1 or 2, wherein said guide
means is constructed to direct specular reflectance out of said
reflectometer.
9. A reflectometer as defined in claim 1, wherein said guide means
is said housing.
10. A reflectometer as defined in claim 1, wherein said guide means
has surfaces extending from said detector means to said positioning
means, angled so that said beam exits one of said surfaces at an
angle of about 90.degree. before illuminating a positioned
element.
11. A reflectometer as defined in claim 2, wherein said guide means
has surfaces extending from said detector means to said test
element support, angled so that said beam exits one of said
surfaces at an angle of about 90.degree. before illuminating said
supported test element.
12. A reflectometer as defined in claim 1 or 2, and further
including shield means disposed around at least a portion of said
detector means for blocking all line-of-sight radiation from said
source means to said detector means.
13. A reflectometer as defined in claim 12, wherein the distance Z
between said shield means and said illuminated test element,
measured from the outside diameter of the light-blocking portion of
said shield means, is the value determined by the equation
Z=w tan .alpha.
wherein w is said outside diameter and .alpha. is the angle at
which said beam is reflected from said reflective surface.
Description
FIELD OF THE INVENTION
This invention relates to a reflectometer constructed to detect
colorimetric densities in a test element.
BACKGROUND OF THE INVENTION
Reflectometers have been constructed featuring optical arrangements
of lenses, filters, apertures, a radiation source, and detector.
Examples are described in U.S. Pat. Nos. 4,219,529; issued Aug. 26,
1980 and 4,224,032, FIGS. 9 and 10; issued on Sept. 23, 1980. In
such arrangements, the separate components, such as the lenses,
have to be accurately located and mounted to insure proper light
ray alignment and focusing.
Although such reflectometers have been successfully used, there has
been a need for a simpler arrangement in which the number of
components is reduced and the positioning of the components
simplified. Particularly, such a need exists in the field of
portable instruments, such as those used by individuals, either at
home or while traveling. For example, in the case of a
reflectometer used as a portable analyzer, there is a need for a
reflectometer that is thin enough to fit in the user's pocket.
U.S. Pat. No. 3,536,927, issued on Oct. 27, 1970, describes a
simplified reflectometer, wherein a light source and a number of
detectors are mounted within a light guide. The light guide acts to
direct the radiation to a plurality of emitting areas, and
radiation reflected by the test object is detected.
Several disadvantages exist in devices such as are shown in the
aforesaid patent. One disadvantage is that no provision is made to
exclude the detection of specular reflectance. Instead, light is
randomly delivered within the light guide at all angles from the
light source, producing radiation that illuminates the test element
at a number of angles. Because the emitted light occurs at such a
variety of angles, encouraged by multiple reflections within the
light guide, no provision can be made to effectively shield the
detector means from specular reflectance. Specular reflection is a
significant problem with test elements that have a transparent
exterior surface, such as a support, that is scanned by the
reflectometer. Examples of elements having such a construction
appear in U.S. Pat. No. 3,992,158, issued Nov. 16, 1976. Such
transparent exterior surfaces specularly reflect about 4% of the
incident radiation, regardless of the absorption of light that
occurs within the test element. Such specular reflection represents
a significant noise factor that must be eliminated in order for
highly accurate readings to be made of low-level analytes.
RELATED APPLICATIONS
Commonly owned U.S. application Ser. No. 337,189, filed on Jan. 5,
1982, by M. Snook and entitled "Fiber Optics Head Featuring Core
Spacing to Block Specular Reflectance" also describes a
reflectometer adapted to exclude specular reflectance from
detection. However, the device described concerns the use of
individual light-transporting fibers, rather than a one-piece light
guide, wherein the spacing between the light-emitting fiber and the
light-receiving fiber is effective to exclude specular reflectance.
Such individual fibers require separate manufacture and subsequent
assembly which can be eliminated by using a one-piece housing which
itself provides the radiation guide means, such as is shown in U.S.
Pat. No. 3,536,927. The difficulty is that the one-piece housing of
the 3 927 patent lacks the desired exclusion of specular
reflectance in the detected radiation.
Therefore, prior to this invention the problem has been to design a
reflectometer having the simplified construction of a one-piece
housing while eliminating the detection of the undesired specular
reflectance.
SUMMARY OF THE INVENTION
As a solution to the above-noted disadvantages and problems, this
invention provides a reflectometer, featuring a one-piece housing,
that is improved to prevent detection of specular reflectance
emanating from the test element.
More specifically, there is provided a compact reflectometer for
the detection of density changes in a test element having a
transparent exterior surface, the reflectometer comprising source
means for illuminating such a test element and detector means for
detecting radiation reflected from a selected portion of the test
element. One-piece molded housing means are included in which said
source means and said detector means are mounted. The housing means
includes means for predeterminedly positioning such test element
and optically transmissive radiation guide means constructed to
guide the radiation to such positioned test element. The
reflectometer is improved in that the source means includes means
for at least partially collimating the beam of radiation and the
radiation guide means includes an internally reflective surface
constructed and located to direct the beam to the positioned test
element. Furthermore, the detector means is located with respect to
said positioning means, the source means and the reflecting surface
so as to receive from an illuminated test element only radiation
diffusely reflected from an illuminated test element.
Thus, it is an advantage of the present invention that the
reflectometer detects reflectance from test elements having an
exterior reflective surface, without detecting specular reflectance
therefrom.
It is another advantage of the present invention that the
reflectometer is portable, because of the compactness of the
elements thereof.
It is a related advantage of the present invention that, because
the reflectometer houses the optical elements in a radiation guide
means, the radiation source can be mounted so as to be wholly
contained within the radiation guide means, thus minimizing the
thickness.
Other features and advantages will become apparent upon reference
to the following Description of the Preferred Embodiments, when
read in light of the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a partially schematic plan view of a reflectometer
constructed in accordance with the invention, used as part of an
analyzer;
FIG. 2a is a fragmentary, vertical section view of the
reflectometer, taken generally along the line II--II of FIG. 1;
FIG. 2b is similar to the view of FIG. 2a, but with section lines
removed and light ray paths added for clarity;
FIG. 2c is similar to the view of FIG. 2b, except that the view is
further simplified to illustrate the relationship between various
distances discussed herein;
FIG. 3 is a fragmentary section view of a test element useful with
the reflectometer of this invention;
FIG. 4 is a section view similar to that of FIG. 2a or 2b, but
illustrating an alternate embodiment; and
FIG. 5 is a graph of concentration plotted vs. reflection density,
detected using the reflectometer of this invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The specific embodiments hereinafter described refer to a
reflectometer that is particularly adaptable (a) as a portable
instrument, (b) for the detection of biological analytes, that is,
components of biological liquids such as serum, and (c) using light
as the illuminating radiation. In addition, the reflectometer of
the invention is useful to test liquids other than biological
liquids, for example, industrial liquids. It is further useful as a
reflectometer used to detect various color densities in
non-biologic test elements, for example, photographic prints.
The analysis of liquids, using the reflectometer of the invention,
is accomplished preferably through the use of generally flat test
elements E, FIGS. 2a and 3, that feature one or more
liquid-containing portions mounted in a plastic frame member. The
liquid-containing portions 12 and 14 are mounted on a transparent,
liquid-impervious support 10, FIG. 3, having an exterior surface
16. The liquid is applied by depositing a quantity, such as a drop,
onto the test element.
The layers of the test elements preferably are constructed in the
manner described in, for example, U.S. Pat. No. 3,992,158, issued
Nov. 16, 1976, and U.S. Pat. No. Re. 30,267, reissued May 6, 1980,
the details of which are expressly incorporated herein by
reference. Deposited sample liquid spreads first into layer 14, a
spreading layer, and then into layer 12. Layer 14 is preferably
constructed to reflect light from its interface 18 with layer 12.
Preferably, layer 12 is a reagent layer and therefore the locus of
the reaction that takes place that generates a detectable change.
U.S. Pat. No. 4,169,751, issued on Oct. 2, 1979, discloses one
useful form of such a test element.
Specular reflection or reflectance from surface 16 of element E is
a noise factor of significant proportions. "Specular reflection" or
"specular reflectance" is used herein in its conventional meaning,
that is, reflection in which "the directions of the incident and
reflected radiation make equal angles with a line perpendicular to
the reflecting surface [usually called the `normal`]", McGraw-Hill
Dictionary of Scientific and Technical Terms (1969). Therefore,
specular reflection is generally to be distinguished from diffuse
reflection, which latter occurs at all angles rather than just the
angle of incidence.
It will be appreciated that the transparent exterior surface 16 of
the test element, although the primary locus of unwanted specular
reflection, is not the only such locus. That is, specular
reflection occurs also at an interface 20 located between the
support layer and the reagent layer, FIG. 3, as follows:
Illuminating radiation 100 directed onto test element E produces
the following reflections. For any given radiation 100 that
impinges onto the surface 16 of transparent support layer 10 at
point A, there is a small fraction of specular reflection 102.
(Line 101 is the normal to surface 16.) There is essentially no
detectable diffuse reflection from point A, by reason of the high
degree of transparency of the support layer. The majority of beam
100 passes through layer 10 to strike interface 20 at point B.
Because there is never a perfect match of indices of refraction,
some specular reflection 104 is emitted from interface 20, along
with a small amount of diffuse reflection schematically indicated
as arrow 106. The remaining amount of radiation 100 attempts to
traverse reagent layer 12 to point C at interface 18 located
between reagent layer 12 and the spreading layer 14. Because, as
noted, the spreading layer 14 is highly reflective, little
radiation proceeds beyond point C. Most of the radiation is
diffusely reflected, schematically indicated in FIG. 3 by arrow
110. As the light passes from point B to point C and back, it
traverses the particles of layer 12. To the extent those particles
are radiation-absorbing dye, the diffuse component 110 is reduced
proportionately. It is this diffuse component that is not absorbed
that is detected as an inverse measure of the amount of dye, and
therefore analyte, that is present. Conversely, specularly
reflected radiations 102 and 104 never traverse the dye particles
of layer 12. Therefore, for best results radiations 102 and 104 are
to be excluded from detection.
There may be a slight amount of specular reflection 112 at point C,
but this reflection can be ignored since it is not likely it will
get past the light-absorbing dye particles that are produced in
layer 12.
It is a characteristic of this invention that radiation 110, but
not specular reflections 102 or 104, is detected as follows:
Reflectometer 30, FIGS. 1 and 2a, comprises a housing 32 which
preferably is itself a light guide, at least one source means 34 of
illuminating radiation having a lens 35, and a detector means, such
as a photodetector 36. "Light guide" as used herein means a device
constructed of optically transmissive material having at boundaries
intended to be internally reflective, a smooth external surface,
such that light is uniformly transmitted within the material
without exiting such smooth external surfaces except along paths
that intersect such surfaces at a relatively steep angle. Any such
radiation guide is useful regardless of whether the radiation is
visible or not, if constructed to similarly direct and transmit
whatever form of radiation is used. In preferred materials, light
radiation enters the light guide and exits only if the exiting path
forms an angle to the surface of the light guide that is at least
about 45.degree.. Various plastics are useful in making such a
light guide, particularly as a molded piece. Methyl methacrylate
available from Rohm Haas Co. under the trademark "Plexiglas", is a
particularly useful material.
Housing 32 is provided with a portion adapted to support a test
element E, shown in dotted lines, FIG. 2a. Specifically, support
surface 38 is provided, preferably recessed below the uppermost
surface 39 of housing 32.
Housing 32 is also provided with receptacles 40 and 42, the latter
being used to mount the photodetector. Preferably, receptacles 40
are cylindrical wells with generally squared off ends 43, FIG. 2b,
each sized to accommodate a single source means 34 wholly within
the housing. Ends 43 need only be generally flat, as occurs in
molded plastic, and only generally perpendicular to axis 48 of beam
47. Thus, slight depressions are easily tolerated in the surface
forming end 43, and such surfaces are useful even if they deviate
as much as 5.degree. from being perpendicular to axis 48. However,
ends 43 are preferably not curved to conform to the surface of lens
35, as such a curvature tends to scatter the collimated beam.
If several source means 34 are used, each preferably is selected to
have a different wavelength of emission. Four such means are shown
in FIG. 1. Any convenient source means is useful, light sources
being preferred. Most preferred because of their size and
wavelength selectivity are LED's. Preferably each LED is provided
with a spherical lens 35, FIG. 2b, that partially collimates the
emitted light into a generally cylindrical beam 47 having an axis
48, discussed further hereinafter. It will be appreciated that the
more complete the collimation of beam 47, the more readily it can
be controlled in the manner described herein.
Particularly useful examples of LED's include those available from
So Li Co., for example those available under the designation
ESBR/SBR 5501.
Receptacle 42 is divided into two portions, a lower portion 44 and
an upper portion 46, and has an axis 45. Lower portion 44 is
preferably cylindrical and sized to receive the photodetector.
Upper portion 46 of receptacle 42 has a generally frusto-conical
surface, except that planar facets 50, FIG. 1, are formed on the
surface where a plane containing diametrically opposite receptacles
40, and portion 44 of receptacle 42, intersects the wall defining
receptacle portion 46. Planar facets are preferred because they
provide a more uniform emission of radiation from the light guide.
Most preferably, facets 50 are angled so that beam 47 exits
therethrough at an angle of about 90.degree..
Depending upon the photodetector that is used, it may project more
or less out the undersurface light guide 32 than is shown in FIG.
2b. Examples of particularly useful photodetectors include
photodiodes available from Vactec, Inc., under the tradename VTB
1113, having an I.sub.L value of 60 .mu.A, and an I.sub.D value of
20 pA at 2 volts.
At least the portion of the housing 32, FIG. 2b, encompassing beam
47 as it traverses from source means 34 to facet 50 of housing 32,
is the light guide. It will be appreciated that, if as is preferred
the light guide comprises housing 32, it provides a housing readily
manufactured as a one-piece molded plastic.
The light guide further includes a reflective undersurface 56,
operatively disposed between the source means 34 and photodetector
36. That is, surface 56 acts to reflect the beam 47, FIG. 2b, from
source means 34 that impinges upon it. To be reflective, surface 56
is provided either with the normal smoothness of the molded plastic
or with a laminated reflective material such as a metal foil. If
only normal smoothness is used, angle alpha, the angle of beam 47
to surface 56, is selected in accordance with the index of
refraction for the material of the light guide. For the preferred
material methyl methacrylate, angle alpha is no more than about
47.8.degree., for smooth surface 56, to insure the light is
internally reflected from, rather than emitted out of, surface 56.
Most preferably, angle alpha is about 40.degree. for methyl
methacrylate. It has been found that such a smooth surface by
itself is effective in providing total internal reflection of the
beam.
A surprising aspect of the reflectivity of smooth undersurface 56
is that it is not adversely affected by contact with most other
surfaces. The only precaution that the operator should take when
using the smooth undersurface 56 as the means for reflecting beam
47, is to keep surface 56 free from contact with a material that
both (a) wets surface 56 and (b) has a higher index of refraction,
e.g., a piece of adhesive tape. Otherwise, the beam 47 will tend to
leak into that material instead of being reflected.
Although light beam 47 is partially collimated by lens 35, a small
fraction of the light may flare out along path X, FIG. 2b. Path X
represents the farthest deviation from beam 47 that is also aligned
with the detecting portion of detector 36. To avoid detection of
such deviating light, a shield 60 is positioned around at least a
portion of photodetector 36. The blocking portion 62 of the shield
has a height selected to be sufficient to block path X, but
insufficient to block beam 47 from reaching element E. The upper
surface of portion 62 of shield 60 is preferably beveled to provide
a frusto-cone of detection for photodetector 36 that coincides
generally with the surfaces of upper receptacle portion 46.
To trap specular reflectance, preferably a light-absorbing material
is placed diametrically opposite each source means 34, in the path
of the specularly reflected beam 47'. That is, generally
cylindrical beam 47 impinges upon test element E to illuminate a
spot area S, FIG. 2b. The angle of beam 47 to the normal, which as
shown coincides with axis 45 of receptacle 44, is
(90.degree.-.alpha.), and it is this same angle at which specular
reflectance beam 47' extends from element E. Beam 47' also reflects
off undersurface 56, at area S'. A particularly useful trap is a
second source means 34 that is turned off when the first
illuminating source means is activated. Preferably, such second
source means is selected to emit radiation of a predominant
wavelength that is different from that of the first-noted source
means.
Undesired beam 47' is not detected by photodetector 36, because
photodetector 36 detects reflected light from element E, and
specifically whole spot area S, as a conical beam confined within a
maximum cone of detection. As noted, such maximum cone of detection
preferably coincides with, or falls inside of, the general
frusto-conical surfaces of upper portion 46 of receptacle 42. As is
apparent from the beam paths of FIG. 2b, the horizontal positioning
of the cone of detection and beam 47' is such that all of beam 47'
passes outside the detection range of photodetector 36. The path of
beam 47' is in turn controlled by the partial collimation of beam
47, and the selected aiming of axis 48 of beam 47. The distance of
reflection point Y of axis 48 on surface 56, FIG. 2b, from
photodetector 36, is selected for control of the aiming. Such
distance varies with the divergence, if any, of beam 47, as well as
with the dimensions of the cone of detection of photodetector 36
and the spacing distance Z of the shield of the photodetector 36
from test element E, measured at the outside diameter of shield
portion 62. Thus, the less the distance Z, the lesser must be the
distance between reflection point Y and photodetector 36 for a
given angle alpha.
Assuming that photodetector 36 is generally centered on spot S,
FIG. 2b, a useful approximation of the relationship between
distance Z and angle .alpha., apparent from FIG. 2c, is
wherein .alpha. and Z are as described above, and w is said outside
diameter. The distance "x" from the outer edge F of spot S" to the
inwardly-extending shield portion 62 is adjusted so that beams 47
and 47' just clear shield portion 62.
The reflectometer of the invention is particularly useful in an
analyzer that further includes a conventional microcomputer 70 and
display means 72, FIG. 1. Because such parts are conventional, they
require no further description. Electrical connection is made from
source means 34 and photodetector 36 to microcomputer 70 via any
suitable connectors 74.
From the preceding, the manner in which reflectometer thickness is
minimized will be apparent. Because the light guide permits the
light path to be folded, the source means 34 is mountable entirely
within the light guide at the side thereof. In the most preferred
embodiment, source means 34 adds nothing to the thickness of the
reflectometer. Thus, the entire reflectometer has a thickness from
surface 39 to 56 that does not exceed about 1 cm. In contrast, if
source means 34 were to be mounted through undersurface 56 to
project light through a hole up to element E along axis 48, FIG.
2b, to illuminate spot area S, the source means 34 could add about
30% to the thickness of the reflectometer.
The trap for the specular reflection need not utilize an absorbing
material. FIG. 4 illustrates an alternate embodiment in which the
trap is constructed to direct the specular reflectance out of the
reflectometer. That is, preferably the light guide is constructed
so that such reflectance harmlessly exits from the light guide.
Parts similar to those previously described bear the same reference
numeral, to which the distinguishing suffix "a" has been added.
Thus, reflectometer 30a comprises a housing 32a, which is itself
the light guide, and source means 34a and photodetector 36a mounted
in receptacles 40a and 42a as before. However, in this embodiment
there is no inactive source means diametrically opposite receptacle
40a to act as a light trap. Instead, side surface 80 of light guide
32a is free of any contained electrical device and is inclined at
an angle beta to undersurface 56a. The value of beta is selected to
insure that specular reflectance beam 74a' exits out of surface 80,
rather than internally reflects from it. The particular value will
again depend upon the index of refraction for the light guide. In
the case of methyl methacrylate, beta is preferably less than
92.degree., and most preferably about 75.degree.. A value greater
than 92.degree. for beta is undesirable because it would tend to
internally reflect the specular reflectance 47a', probably back to
photodetector 36a.
EXAMPLE
FIG. 5 illustrates the ability of the reflectometer of this
invention to detect varying degrees of concentration as a function
of inversely proportional densities produced in appropriate test
elements. Specifically, test elements constructed as described in
U.S. Pat. No. 3,992,158 were spotted with a 10 .mu.l drop of
calibrator liquid containing twelve different known levels of
concentration of glucose. The densities measured as D.sub.R were
noted, using a reflectometer constructed as shown in FIGS. 1 and
2b, and plotted for those concentrations. The calibrator curve of
FIG. 5 was the result, demonstrating that a proportionally
increasing density was detected at increasing concentrations of
glucose.
The invention has been described in detail with particular
reference to preferred embodiments thereof, but it will be
understood that variations and modifications can be effected within
the spirit and scope of the invention. For example, the invention
is also applicable to a reflectometer that is not portable.
* * * * *